In order to maximise the storage area of a warehouse, it is desirable to make the aisles of the minimum width possible. The aisles must however be wide enough to permit the manoeuvring of fork lift trucks to deposit a load in or remove a load from the stacks.
In order to improve the manoeuvrability of the fork lift trucks and thus reduce the aisle width, GB 2234214, the disclosure of which is incorporated herein by reference thereto, discloses a fork lift truck with two parts that are pivoted together. The rear part comprises a truck body which carries the driver, propulsion unit and counterweights to balance loads carried by a lifting mechanism mounted on the front part. A pair of driven wheels are provided on the truck body and a pair of non-driven wheels are provided on the front part, as close as possible to the load bearing part of the lift mechanism. The truck is steered by turning the front part relative to the truck body, about the pivot axis.
In order to permit loads to be deposited or removed from the stacks at right angles to the aisles, the front part is preferably capable of being turned at 90° or more to the truck body. As the front wheels approach 90°, the drive from the rear wheels will cause the front wheels to slide sideways along the aisle, rather than steering the truck towards the position in the stack into which a load is to be deposited or from which a load is to be removed.
In order to overcome this problem, it has been proposed, for example as disclosed in GB 2263088 or GB 2255941, the disclosure of which is incorporated herein by reference thereto, to provide differential drive to the rear wheels, in order to produce a steering effect.
A more effective approach, as disclosed in GB 2265344 and EP 1201596, the disclosure of which is incorporated herein by reference thereto, has been to drive the front wheels, instead of or in addition to the rear wheels. However in order to provide stability, it is necessary for the weight distribution in this type of lift truck to be very much to the rear of the truck. It is consequently necessary with front wheel drive systems of this type, to provide an articulated front axle to ensure that both front wheels remain in driving engagement with the floor, in spite of irregularities in the floor surface. This will generate further stability problems, particularly with elevated loads and in practice articulation of the front axle must be limited to provide a maximum upward and downward movement of each wheel, of about 25 mm. Even when the front axle is articulated in this manner, wheel spin is libel to occur if there are variations in the floor surface in excess of 20 mm in 1.5 m, which is typical for a newly laid warehouse floor.
According to one aspect of the present invention, a fork lift truck comprises a truck body, a lift mechanism connected to the truck body by means of a vertically extending pivot and means for turning the lift mechanism relative to the truck body about said pivot to steer the truck, the truck body having a pair of rear ground engaging wheels mounted on transverse axes, the lifting mechanism having a single ground engaging front wheel mounted centrally on a transverse axis, characterised in that the front wheel has independent drive means.